Independent eye movement

How it works

Human eyes are yoked together. When you look left, the brain sends the same command to both eyes, so both turn left by the same amount. This rule is called Hering’s law of equal innervation². It keeps both eyes pointed at the same object, which is what two-eyed depth vision needs.

There is one built-in exception. When you look at something near, the eyes turn toward each other. This is vergence, and it moves the eyes in opposite directions by equal amounts¹.

Independent eye movement combines the two. If you turn both eyes to the right and converge at the same time, the leftward pull on the right eye cancels its rightward turn. The right eye stays still while the left eye moves. People who do the trick well have learned to run these two systems at once and balance them. Very few people can do this cleanly. No study has counted them.

Chameleons have fully separate control of each eye. Each eye scans on its own, and the brain joins the two views only when the animal locks onto prey³.

Try it

This is a partial version. It uses the near-object trick and does not need special ability.

1. Hold a finger about ten centimetres in front of your nose, slightly to the right.
2. Look at the finger. Your right eye turns outward and your left eye turns hard inward.
3. Move the finger slowly to the right, keeping it close to your face.
4. Your right eye follows it outward. Your left eye stays turned in toward the nose and barely moves.
5. Film it on your phone. The effect is clearer on video than in a mirror.